In satellite communication systems, reception at low signal to noise ratio (SNR) is evident when the receiving antenna is small and/or the signal being received is transmitted at limited (low) power. Spreading techniques are well known to enable reliable communication under low SNR conditions. In one such spreading technique, known as repetitions, each symbol is transmitted several times and the repetitions are coherently combined at a receiver. Thus, while each transmitted instance of a symbol is received at a low SNR, the SNR of a symbol combined from all its repetition instances is high enough to enable reliable communication.
In one method for transmitting symbol repetitions, known as block repetition, successive repetitions of an entire block of symbols are transmitted. Carrier frequency offsets between a transmitter and a receiver cause the phase of received symbols to rotate with time. Consequently, phase offsets appear between repetitions of the same symbol, making simple (non-coherent) combining of repetitions of the same symbol ineffective. Therefore, when combining repetitions it is necessary to first estimate the phase offset between the combined repetitions, to cancel the estimated phase offset and only then to combine (sum) the repetitions (coherent combining).
In one approach towards coherent combining of repetitions, phase offset estimation and combining the phase-corrected repetitions is done gradually. First, a phase offset between the first repetition and the second repetition is estimated, the phase of the second repetition is corrected in accordance with the estimated phase offset (i.e., the phase offset is canceled) and the two repetitions are summed (combined), resulting in a first combined repetitions result. Then, the phase offset between the first combined repetitions result and the third repetition is estimated, the phase offset of the third repetition is canceled and the third repetition is added to the first combined repetitions result, resulting in a second combined repetitions result. In case there are additional repetitions, the process described above continues in a similar manner until all the repetitions are combined.
However, the method described above (i.e., block phase offset estimation, phase-offset cancelation, and combining) yields poor results when the signal is received at low SNR. The fundamental weakness of that method is the phase estimation accuracy. Since the phase estimations are performed in stages, wherein at each stage the phase offset is estimated using only a relatively small portion of the received signal's energy, the estimation accuracy is significantly degraded when the signal is received at low SNR. With the phase estimations being significantly inaccurate, the combining of the repetitions becomes non-coherent and thus ineffective.